Genome-Wide Identification of Sigma Factors in Brassica napus and Role of BnSIG5A in Response to Cold Stress

Sigma factors (SIGs) are nuclear-encoded regulators of chloroplast gene transcription. We conducted a genome-wide analysis in Brassica napus, identifying 23 SIG genes that were phylogenetically classified into six distinct subfamilies. Characterization of gene structure, conserved motifs, and chromosomal locations indicated family expansion primarily through segmental duplication under purifying selection. Promoter analysis identified cold-responsive elements enriched in BnSIG5A. Expression profiling showed that BnSIG5 subfamily members, particularly BnSIG5A, are strongly induced by cold stress. Analysis of Arabidopsis SIG5 mutants confirmed previously reported roles of AtSIG5 in cold tolerance. Heterologous expression in yeast, and the strong cold induction of BnSIG5A together with its chloroplast localization, suggest that BnSIG5A may play a conserved role, providing a foundation for future functional studies in B. napus. This work establishes a genomic framework for the SIG family in rapeseed and identifies BnSIG5A as a high-priority candidate for further investigation. Subcellular localization confirmed chloroplast targeting of BnSIG5A. Heterologous expression in yeast and analysis of Arabidopsis SIG5 mutants suggest conserved functions in cold tolerance, providing a foundation for future functional studies in B. napus. This work establishes a genomic framework for understanding SIG-mediated stress responses in rapeseed and identifies BnSIG5A as a promising candidate for further investigation.